In order for a plurality of hydraulic actuators to be supplied with a pressurized discharge fluid from a single hydraulic pump, a discharge path of the single hydraulic pump can be provided with a plurality of operating valves in parallel to one another so that by switching over the operating valves the respective hydraulic actuators may be supplied with the pressurized fluid. If such an arrangement is adopted, however, when a plurality of the hydraulic actuators should be supplied with the pressurized fluid at the same time, it will follow that the pressurized fluid may be supplied only to an actuator whose load is small and may not be supplied to an actuator whose load is large.
Designed to resolve this problem, there has hitherto been known a pressurized fluid supply apparatus as shown in FIG. 1.
Specifically, as shown in FIG. 1, the hydraulic pump 10 will, by changing the angle of inclination of a swash plate 11, be rendered a hydraulic pump of variable displacement type in which its displacement, that is, its discharging flow per one revolution, is varied, and in which the swash plate 11 is inclined by means of a large diameter piston 12 in a displacement decreasing direction and is inclined by means of a small diameter piston 13 in a displacement increasing direction.
The above mentioned large diameter piston 12 has a pressure receiving chamber 12a which in the sense of communication is connected to and disconnected from a discharge path 10a of the hydraulic pump 10 by way of a switching valve 14, whereas the above mentioned small diameter piston 13 has a pressure receiving chamber 13a which is connected to the above mentioned discharge path 10a.
The discharge path 10a of the above mentioned hydraulic pump 10 is provided with a plurality of directional control valves 15. In each circuit 17 for connecting each of the directional control valves 15 to each hydraulic actuator 16, there is provided a pressure compensation valve 18, respectively. And they are constructed in such a manner that the said pressure compensation valve 18 may be thrusted towards a low pressure setting side under a pressure effective at a first pressure receiving portion 19 and may be thrusted towards a high pressure setting side under a pressure effective at a second pressure receiving portion 20. The first pressure receiving portion 19 is connected to an outlet side of each directional control valve 15 so as to be applied with an outlet side pressure that is effective therein, whereas the second pressure receiving portion 20 is connected to the respective circuit 17 via a shuttle valve 21 so as to be applied with the highest load pressure that is effective therein.
The above mentioned switching valve 14 is thrusted in the direction of communication under a pressure effective within the discharge path 10a and is thrusted in the direction of drain both by a spring 22 and under the above mentioned load pressure. When the discharge pressure P1 becomes higher than a force applied by the spring 22, the switching valve 14 will be shifted to apply the discharge pressure to the pressure receiving chamber 12a of the large diameter piston 12, thereby inclining the swash plate 11 in the displacement decreasing direction. When the discharge pressure P1 becomes lower than the force applied by the spring 22, the switching valve 14 will be returned to its original position to cause the the pressurized fluid in pressure receiving chamber 12a of the large diameter piston 12 to flow out into the tank side, thereby inclining the swash plate 11 in the displacement increasing direction.
A circuit 23 for applying the above mentioned load pressure to the pressure receiving portion 14a of the switching valve 14, that is, the circuit 23 for connecting the pressure receiving portion 14a to the output side of the shuttle valve 21, is connected to a tank 25 via a restrictor 24.
With such a pressurized fluid supply apparatus, if each directional control valve 15 is switched from its neutral position a to a supply position b, the pressurized discharge fluid from the hydraulic pump 10 will be supplied to the hydraulic actuators 16 while their higher load pressure will be detected at the shuttle valve 21 to act on the second pressure receiving portions 20 of the pressure compensation valves 18, respectively. Since each pressure compensation valve 18 is then set under the highest load pressure, the pressurized discharge fluid from the hydraulic pump 10 can be supplied to a plurality of the hydraulic actuators which are of different loads.
In such a pressurized fluid supply apparatus, an arrangement is adopted such that the highest load pressure detected at the shuttle valve 21 may act on the pressure receiving portion 14a of the switching valve 14 to operate the switching valve 14, thereby so controlling the displacement of the hydraulic pump 10 that a pressure difference between the pump discharge pressure P1 and the load pressure P.sub.LS can be maintained constant at all times.
This being the case, the flow supplied to a said hydraulic actuator 16 is determined by the meter-in opening area of a said directional control valve 15 associated therewith. If a large volumetric flow is supplied into a particular actuator 16, for example, a boom cylinder or an arm cylinder in a power shovel, it is necessary that a particular directional control valve associated therewith be large-sized to enlarge the meter-in opening area thereof, or that the above mentioned pressure difference between the pump discharge pressure P1 and the load pressure P.sub.LS be made larger.
However, if such a directional control valve 15 is large-sized, it ought to become different in size from another directional control valve 15 and this is disadvantageous in that they cannot be commonly used and it will necessarily increase their overall cost.
Also, in the pressurized fluid supply apparatus, the pressure loss due to the return flow that is flushed out of a said actuator 16 into a tank is determined by the meter-out opening area of the associated directional control valve. Hence, if a large volumetric flow is flushed out into the tank from a particular actuator 16, for example, a boom cylinder or an arm cylinder in a power shovel, the directional control valve associated therewith should be large-sized to enlarge the meter-out opening area, thereby reducing the pressure loss.
Especially, if the boom cylinder or the arm cylinder is contractively operated, the pressurized fluid is supplied into the contracting chamber of the cylinder, and the pressurized fluid within the elongating chamber is flushed out into the tank. Then, since the area of the elongating chamber of the cylinder is larger than that of its contracting chamber and the volumetric flow flushed out of the elongating chamber into the tank is more than the volumetric flow supplied into the contracting chamber, it is necessary that a large volumetric flow be flushed out of the elongating chamber into the tank in order for the boom cylinder or the arm cylinder to be contractively operated quickly.
However, if the said directional control valve 15 is large-sized, it becomes different in size from another directional control valve 15 to the disadvantage that they cannot be commonly used and an increased overall cost does result.